Method for bonding at least two tissue papers to each other

ABSTRACT

A method for autogenously bonding at least two plies of tissue paper with a basis weight from 10 to 45 g/m 2  comprises the steps of: laying the plies face-to-face; ultrasonically bonding the plies to each other with the help of at least one ultrasonic horn which oscillates perpendicular to the surface to the tissue paper plies. A multi-ply tissue paper produced by the method is also described.

TECHNICAL FIELD

[0001] This invention relates to a method for autogenously ply-bondingat least two tissue paper plies and a multiply tissue paper ply-bondedwith the method.

BACKGROUND OF THE INVENTION

[0002] Tissue paper are often used to absorb liquids in hygienic areas.In order to improve the absorption of tissue paper more than one tissuepaper ply is often combined to a multiply product. When more than oneply is used, it is advantageous to bond the plies together. It has longbeen known to bond tissue paper with adhesive and different types ofmechanical ply-bonding methods.

[0003] WO 96/32248 is one of the many patents describing the use ofadhesives for ply-bonding a multiply tissue paper. The multiply papercomprises a first ply of absorbent paper web having a first pattern ofprotrusions and a second ply having a second pattern of protrusions. Thesecond ply is foot-to-foot laminated to the first ply. The adhesive ispositioned between the first and second plies to selected protrusions,in a predetermined decorative pattern. However, the use of adhesivetends to make the product stiff. The choice of adhesives is limited asthe adhesive should preferably be water soluble to not disturb theproduction process. The adhesive should not react with any of thematerials it are exposed to during its use nor it should it be harmfulto the user. Further the use adhesive is expensive both because of thecost of the adhesive and the fact that the production apparatus has tobe more complicated than if no adhesive had been used.

[0004] Mechanical ply-bonding is an alternative to adhesive ply-bonding.High-pressure mechanical ply-bonding, knurling or edge embossing andperforation embossing are some different mechanical ply-bondingtechniques. EP-A-0,672,402 describes one way of mechanical ply-bonding amultiply tissue paper with a method that includes the steps of: 1)providing at least two superposed plies of tissue paper; 2) passing thesuperposed plies through a crimp roll arrangement including a rotatingcrimp element and a rotating anvil element and 3) applying a pressureload against the rotating elements sufficient to crimp bond the plies.However, the mechanical ply-bonding achieves a fairly week bonding inthe multiply tissue paper, whereby the laminate easily delaminates.Further, a mechanical ply-bonding often gives the paper an unattractiveappearance especially if knurling is used.

[0005] It is known to use ultrasonic means for bonding paper materials,e.g. teabags, silk paper and package paper.

[0006] CH 399,988 and DE 11,56,303 describes ultrasonic bonding of paperfor teabags and silk paper, a very, thin and smooth type of paper,respectively. The ultrasonic welding horn oscillates parallel to thesurface of the treated paper. The person skilled in the art for tissuewould not transfer the teachings from DE 11,56,303 and CH 399,988 totissue paper as the paper quality is another The tea bag paper and thesilk paper is planar, while tissue paper due to creping is bulky. Due tothe creping an ultrasonic horn oscillating parallel to the paper surfacewould rub the tissue paper surface and thereby damage or even destroyit.

[0007] WO 99/25547 describes a method of ultrasonic welding of packagingpaper where water is added to the bonding surfaces before the ultrasonictreatment. At least 80% of the water applied will be removed during theultrasonic treatment. The method is unsuitable for bonding tissue paperbecause of the high absorbency and wicking effect of the tissue paper.Due to these features the tissue paper will absorb the water and lead itaway from the ply-bonding region so that the water concentration inply-bonding region will be so low that the added water has no or just aninfinitesimal effect. Another disadvantage with the wetting of the paperwould be that a creped paper would loose its crepe wrinkles and therebyits bulk. Further, the paper would have to be dried in a subsequent stepas the water that has been lead away from the bonding region will no beeffected by the ultrasonic treatment and will therefor not evaporate butwill stay in the paper. The problem with the low water concentration inthe ply-bonding region could of course be solved in that more water isapplied to the tissue paper. However, this added water increases theamount of water that has to be removed in the subsequent drying step.Therefor would the method described in WO 99/25547 be uneconomical inthe production of tissue paper and would therefor not be used.

[0008] The object of the invention is to achieve a cheap tissue paperbonding method that at the same time provides good ply-bonding strengthin the produced multi-ply tissue paper. A further object is toaccomplish the bonding without the help of adhesive or thermoplasticmaterial.

SUMMARY OF THE INVENTION

[0009] This object is achieved in accordance with the present invention,by means of a method for autogenously bonding at least two plies oftissue paper with a basis weight from 10 to 45 g/m² comprises the stepsof laying the plies face-to-face, bonding the plies to each other withthe help of ultrasonic means, the at least one ultrasonic horn of theultrasonic means oscillates perpendicular to the surface to the tissuepaper plies

[0010] According to one preferred embodiment of the inventive method thetissue paper plies are dry during the bonding procedure. In an even morepreferred embodiment the fibres the tissue paper are made of purelycellulosic fibres. It also preferred that at least one of the plies ischosen from the group of dry-creped tissue paper, through-air-driedtissue paper and impulse embossed dried tissue paper.

[0011] During the bonding, the ultrasonic means oscillatesperpendicularly to the surface of the tissue paper plies with afrequency from 15 to 50 kHz, preferably from 20 to 40 kHz, mostpreferably 35 kHz.

[0012] The invention is further characterised by a so called bondingarea. The bonding area is the sum of the areas of the bonds in theply-bonding zone. The ply-bonding zone consists usually of patterns ofbonds. The bonds may be discrete spots or continuous lines or continuousnetwork distributed in a pattern within the ply-bonding zone. A tissuepaper according to the invention could of course have a bonding areathat totally covers the area of the ply-bonding zone, i.e. a bondingarea of 100%. However, such a paper could be quite stiff an unpleasantto use. Therefore, in a preferred embodiment of the invention thebonding area is from 0.5 to 50, preferably from 2 to 40% and mostpreferred from 5 to 20% of the area of the ply-bonding zone. Thisresults in a tissue paper where the tissue plies can move slightly inrelation to each other between the bonds which increases the feel of asoft paper.

[0013] Some tissue paper qualities e.g. household towels, usually havebonds in the form of spots distributed all over the surface of thetissue paper more or less regularly. In this case, the ply-bonding zonecovers the whole tissue paper. Other tissue paper qualities, e.g.handkerchiefs, napkins and facial tissues are usually ply-bonded only innarrow zones along two or more outer side of the finished product. Inthese cases the ply-bonding zone consist just of this/these narrowzones. (The rest of the surface of the product is not included in theply-bonding zone.)

[0014] A further preferred embodiment of the invention is to use apressure force between the ultrasonic means and the tissue paper pliesto be bonded that is from 50 to 700 N, preferably from 200 to 500 N.

[0015] An embodiment of the method according to this invention is thatthe tissue paper plies are ultrasonically bonded while they aretransported at speed from 50 to 1800 m/s, preferably, from 300 to 800m/s.

[0016] In a further preferred embodiment lotion is applied to the tissuepaper that has been bonded with the ultrasonic method. Tissue paper withlotion are difficult to ply-bond. The bonds achieved with commonly usedsystems with adhesive or mechanical ply-bonding means, are not strongenough and tend to break up if a lotion is applied to the paper.However, it has surprisingly been found that the ultrasonic methodaccording to this invention achieves a bonds strong enough to withstandthe strain when lotioning the tissue paper plies after the ply-bondingstep. Further, the ultrasonic bonded tissue paper plies withstands thedelamination effect of a fatty and/or moist lotion without weakeningplybonding in contrast to common mechanical or adhesive bonds.

[0017] The invention also relates to a multiply tissue paper that hasbeen bonded autogenously by ultrasonic bonding. Each ply of the tissuepaper has a basis weight from 10 to 45 g/m². It is further preferredthat the paper is dry during the bonding operation. In a preferredembodiment, the fibres of the tissue paper are made of are purelycellulosic fibres. It is also preferred that at least one of the pliesis chosen from the group of dry-creped tissue paper, through-air-driedtissue paper and impulse embossed dried tissue paper. The multiplytissue paper is preferably bonded according to the method embodimentsdescribed above.

[0018] The ply-bonding zone consists of a pattern of bonds. The bondsmay be discrete spots or continuous lines or continuous networkdistributed within the ply-bonding zone. A tissue paper according to theinvention could of course have a bonding area that totally covers thearea of the ply-bonding zone, i.e.a bonding area that is 100% of theply-bonding zone. However, such a paper could be quite stiff anunpleasant to use. Therefore, in a preferred embodiment of the inventionthe sum of the areas of the bonds is from 0.5 to 50%, preferably from 2to 40% and most preferred from 5 to 20% of the area of the ply-bondingzone. This will give a tissue paper where the tissue plies can moveslightly in relation to each other between the bonds, which increasesthe perception of a soft paper.

[0019] A tissue paper product may have ply-bonding zones that covers thetissue paper product either totally or just partially. Household towelsare one example of a tissue paper product where the entire productsurface usually is a ply-bonding zone. A household towel usually havebonds in the form of spots distributed all over the surface of thetissue paper in a more or less regular pattern. Other tissue paperqualities, e.g. handkerchiefs, napkins and facial tissues are usuallyply-bonded only in narrow zones along two or more outer side of thefinished product. In these cases the ply-bonding zone consist just ofthis/these narrow zones. (The rest of the surface of the product is notincluded in the ply-bonding zone.)

[0020] A further preferred embodiment is an ultrasonically bonded tissuepaper according to this invention to which lotion has been applied afterthe bonding step. Tissue paper with lotion are difficult to ply-bond.The bonds achieved with commonly used systems with adhesive ormechanical ply-bonding means, are not strong enough and tend to break upif a lotion is applied to the paper. However, it has surprisingly beenfound that the ultrasonic method according to this invention achieves abonds strong enough to withstand the strain when lotioning the tissuepaper plies after the ply-bonding step. Further, the ultrasonic bondedtissue paper plies withstands the effect a fatty and/or moist lotionwithout weakening in contrast to common mechanical or adhesive bonds.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The invention will now be described with reference to theenclosed figures, of which;

[0022]FIG. 1 shows an schematic illustration of an apparatus forperforming the method according to the invention,

[0023]FIG. 2 shows a detailed view of the apparatus in FIG. 1,

[0024]FIG. 3 shows a side view of the apparatus in FIG. 1 with a webpassing through the gap,

[0025]FIG. 4 shows how ply-bonding strength samples may be taken from apaper that is bonded all over the surface,

[0026]FIG. 5 shows how ply-bonding strength samples may be taken from aproduct bonded along the border,

[0027]FIG. 6 shows how ply-bonding strength samples may be taken from aproduct that is bonded in narrow strips,

[0028]FIG. 7 shows the clamping configuration when measuring ply-bondingstrength, and

[0029]FIG. 8 shows an example of a bonding pattern for tissue paper.

DETAILED DESCRIPTION OF THE INVENTION

[0030] In FIG. 3 is schematically shown an apparatus for autogenouslybonding at least two tissue plies 5′,5″, each having a basis weight from10 to 45 g/m² together. The apparatus 1,3 comprises means for feeding(not shown) two plies of tissue paper 5′,5″ in a face to faceconfiguration through the apparatus. It further comprises at least oneultrasonic horn 1, which forms a bonding gap 2 with a structured anvil3. Through this gap the tissue paper plies are fed. The ultrasonic horn1 oscillates perpendicularly to the anvil surface 3. The apparatus alsohave means 18, e.g. a spray nozzle, for applying a lotion to the bondedplies after the passage of the ultrasonic means 1,3.

[0031] In the shown example the apparatus comprises a ultrasonic horn 1,but also other kinds of ultrasonic means such e.g. a rotating disc maybe used. Such a disc can be used for applying a thin strip of continuousplybonding.

[0032] The anvil 3 has a width in the cross machine direction, the widthbeing identical to or greater than the width of the tissue paper plies5′,5″. A preferred embodiment of such an anvil is a rotatable roll.

[0033] The structured anvil has a structured surface with protrusion 4protruding from the surface. A normal structured embossing rolls forembossing tissue paper can be used as anvil roll. The anvil roll may bemade of steel or other metals.

[0034] Instead of having only one ultrasonic horn that covers all thewidth of the anvil, it is preferred to have more than one horn beinglocated side by side along the width of the anvil. It is preferred tohave from 5 to 20 horns per meter.

[0035]FIG. 1 shows details of the bonding apparatus according to theinvention. Two ultrasonic means in the form of ultrasonic horns 1 form abonding gap 2 with a structured anvil roll 3. The two ultrasonic hornsare placed next to each other and covers the entire width W of anvilroll. It would of course be possible to use more horns if the apparatuswould be wider or the horns less wide. In FIG. 2, it is easy to discernthe protrusion 4 on the structured anvil roll. The protrusions have twodifferent designs, evenly and alternately distributed over the surfaceof the anvil roll. The anvil roll is rotatable mounted in the bondingapparatus. FIG. 3 describes how the tissue paper plies 5′, 5″ can belead through the bonding apparatus. The ultrasonic horns oscillatesperpendicularly to the anvil surface in the gap.

[0036] In FIG. 1 the ultrasonic means are depicted as ultrasonic horns.However all other suitable kinds of ultrasonic means may be used, suchas a rotating ultrasonic disk.

[0037] Even though the anvil cylinder has two patterns it possible tomake anvil cylinders with just one type of patterns or with more thantwo patterns if that would be wanted.

[0038] The cylinder has a smooth surface except where the protrusion 4are disposed. Two tissue paper plies 5′, 5″ are fed to the apparatus andlaid face-to-face before the bonding gap 2 of the bonding apparatus. Inthe bonding gap 2, the plies will be bonded to each other withultrasonic bonding between the ultrasonic horn 1 and the anvil roll 3.In this specific apparatus the anvil roll will rotate with the samespeed as the tissue paper plies. In other types of machine the anvilroll may rotate in another speed than the tissue paper.

[0039] In this application, “a dry tissue paper” is a tissue paper whichhas a humidity content that is in equilibrium with the surroundingenvironment, usually air.

[0040] “Autogenously bonding” is in this application defined as abonding of surfaces without the use of adhesive or thermoplasticmaterial.

[0041] Tissue paper is essentially made of fibres of vegetable origin,especially of cellulosic fibres. Typical for tissue paper is its hightensile energy absorption index, in connection with its comparativelylow basis weight the basis weight being from 10 to 45 g/m². Anothercommon feature of tissue paper is its relatively high liquid absorbency.Most of the functional properties specific for a tissue paper, such assoftness, drapability, liquid absorption, bulk softness, and thicknessare related to the tensile energy absorption index.

[0042] Tensile energy absorption index is the volume specific tensileabsorption, whereby the volume (length×width×thickness) of the testsample is decided before the measurement.

[0043] The tensile energy absorption index results from the wet or dryforeshortening of the paper web during the production process. Thefibres forming the web are slurried in water. The resulting very thinfibre slurry is then introduced into the paper machine on a screen orbetween two screens forming a wet web by dewatering the fibre slurry.Another way of forming the web is to introduce the fibres onto thescreen in the form of a aqueous foam and then dewater the foam forming awet web. The so formed web can be then foreshortened by varioustechniques. Below are the most common foreshortening techniquesdescribed.

[0044] One technique used in the paper machine is based on differentspeeds between two consecutive fabrics between which the wet web istransferred. The first faster fabric may be the forming screen or atransfer fabric, while the second slower fabric may be another transferfabric or a drying fabric, e.g. an imprinting or TAD-fabric. When thepaper way is transferred from the fast first fabric, the fibres willaccumulate because of the slower speed of the second fabric. Thisaccumulation results in an internal breaking up of the still moist,plastically deformable web

[0045] Another foreshortening technique relating to the wet web is toadhere the still wet web to a heated wet creping cylinder in the papermachine. On this cylinder the web is dried to a dryness of about 45-80%.Then the web is removed from the cylinder by a doctor blade. Beforebeing adhered to the cylinder the web usually has a dryness of at least30-50%.

[0046] The most common way to foreshorten a paper web in a paper machineis to adhere it to drying cylinder, e.g. a Yankee cylinder, at a drynessof at least 30-50% and to dry it until it is dry, i.e. to a dryness ofat least 85-100%. The dried web is then removed from the cylinder with adoctor blade. Thereby the paper is creped and forms many crepe wrinkles.The resulting paper has a higher bulk and thickness than the web adheredto the drying cylinder. It is also more absorbent and has a higherdrapability than an uncreped paper would have.

[0047] The dried paper can also be creped outside the paper machine. Thedried web is then adhered to a cylinder whereafter it is removed fromthe cylinder with a doctor blade. The cylinder may be heated.

[0048] There are different ways of moulding and drying the wet web in apaper machine. In an conventional tissue machine the web can be pressedafter it has been removed from the forming screen to remove the amountof water left in it Thereafter the web is adhered to the Yankee cylinderas described above and removed by a doctor blade.

[0049] In the more modern through-air-drying technique (TAD), the web ispartially or wholly dried with hot air that is blown through the paper.During this process, the web can be supported by an imprinting- orTAD-fabric. This TAD-fabric may have a distinct three dimensionalpattern into which the web can be moulded before the TAD. The web isoften transferred from the forming screen to the TAD-fabric with therush transfer described above. The web may also be subjected to a finaldrying on a heated cylinder from which it may be removed with or withoutcreping. The web is usually pressed against the heated cylinder when itis still supported on the TAD-fabric to further imprint the structure ofthe TAD-fabric to the paper.

[0050] The imprinting method described above may also be incorporated inthe conventional drying process described above.

[0051] Another way of drying the web is impulse embossing drying wherebythe web is heated up rapidly under high pressure. When the pressurethereafter is released the heated water evaporates and expands rapidly,almost explosively. The so formed paper often have a strong thethree-dimensional pattern and a high bulk.

[0052] According to the invention tissue paper is made of purelycellulosic fibres, e.g. ligno-cellulosic fibres made from wood orregenerated cellulose fibres such as viscose or lyocell. Theligno-cellulosic fibres may be made by any common chemical,chemo-mechanical, chemothermal-mechanical or mechanical method.

[0053] The tissue paper coming from the paper machine is often calledraw tissue paper to distinguish it from the tissue paper in the finalproduct. The raw tissue paper can be converted to the final process inmany ways, e.g. through embossing, laminating, and rolling

[0054] The raw tissue paper coming from the tissue paper machine is asingle ply paper. This single ply paper may comprise more than onelayer. The layers in a multilayered tissue paper have been formed eitherwith a multi-layered headbox, by forming a new layer on an existentlayer or by couching together formed still wet layers. These layers cannot or just with considerable difficulties be separated from each otherand is joined to each other mainly by hydrogen bonds. The layer may beidentical or may have different properties, regarding e.g. fibrecomposition and chemical composition.

[0055] Tissue papers is used in hygienic products personal grooming andhygiene, in the household sector, industry and the institutional filedfor cleaning processes. They are used to absorb fluids. The tissuepapers is e.g. used as toilet paper, hand towels, household towels,handkerchiefs, facial tissues and napkins. The can also be used asindustrial wipes.

[0056] The tissue paper may be treated with lotions to enhance thesoftness and to impregnate the paper with skin treating agents. Withlotion is meant all types of lotion, emollients and softener that areapplied to the dry tissue paper. Most of these lotions have fattyingredients. Examples of such lotions can be found in EP-A-1,029,9797,U.S. Pat. No. 4,513,051, WO-A1-94/29521, WO-A1-00/64409 andWO-A-97/30217, as well as in the co-pending applications EP 01101136.8and EP01101137.6.

[0057] Test Methods

[0058] Basis Weight

[0059] The basis weight is measured in that a climate conditioned squaretest sample with 10 cm sides are cut and weighed. The paper have beenclimatised for 24 hours in a temperature of 23° C. and a humidity of50%.

[0060] Ply-Bonding Strength

[0061] Ply-bonding strength is the average of the tensile force of allpeaks exceeding the mean force. It is determined with a tensile testeroperating at a constant elongation rate. The tensile testerautomatically measures and records the tensile force as a function ofthe testing path. The ply bonding strength is calculated based on thesedata.

[0062] The tensile tester according to the EN ISO 1924-2 (DIN 51221 Part1, Class 1) shall be appropriate for testing multi-ply tissue productswith specified dimensions at a constant elongation rate of (10012)mm/min. It shall be appropriate to record the tensile force as afunction of the path on a strip chart recorder or any similar device. Inthe test a tenslie tester Z2.5/TN1s from Zwick GmbH was used.

[0063] The tensile tester shall record the values for the path to 0.1mm. Measuring should start following an initial stress of 0.01 N. A 10 Nload cell should be used.

[0064] The tensile tester shall have two clamps capable of holding asample 50 mm wide. Each clamp shall be designed so that the sample isheld tightly without causing damage along a straight line over theentire width (test span line). The weight of the upper clamp attached tothe load cell should not exceed 10 g. The clamp lines shall remainparallel to each other during the test. In addition, these clamp linesshall be at a right angle to the tensile force applied and to thehorizontal direction of the sample. The distance between the clamp lines(test span length) shall be adjustable with an accuracy to ±1 mm.

[0065] The cutting device for preparing the samples shall comply withthe requirements of EN ISO 536. It shall allow samples of (50.0±10.5) mmto be cut with undamaged, straight, smooth and parallel edges.

[0066] As specified in EN 20187, the samples shall be conditioned for 24hours in a climate with a temperature of 23° C. and a humidity 50% andkept in that climate during the test.

[0067] Each sample shall have a width of (50±0.5) mm. The length ofsample strips shall be 100 mm in the case of full-surface bonded samplesand 60 mm in the case of samples, which are bonded along the border orjust in narrow stripes, e.g. with edge embossing. The samples may nothave any holes or flaws. With the exception of tissue paper or tissueproducts with embossing covering the surface partially or fully, thesamples shall not have any folds, embossing, breaks or otherfluctuations in thickness.

[0068]FIG. 4 shows how the samples 10 may be cut from a paper that isbonded with full-surface bonding. As can be seen from FIG. 4 the samplescan be cut so that the long side of the samples is aligned with themachine direction (MD) and the short side is aligned with the crossmachine direction (CD). Other alignments of the samples are alsopossible, e.g with the long side of sample being parallel to CD. Theblack points represent bonds 11.

[0069]FIG. 5 shows how samples 10′ may be cut from a paper bonded alongthe border, e.g. a handkerchief or a napkin. The samples can be pulledin both machine direction (MD) and cross direction (CD). The ply-bondingzone 12 at the border is covered with bonds 11. The samples are cut sothat a long side of the ply-bonding zone of each sample will beperpendicular to the pulling direction. Further the samples are cut sothat no folds 13 of the tissue paper are included in the samples.

[0070]FIG. 6 shows how samples 10″ may be cut from tissue paper sheetsthat are bonded in thin stripes 14, e.g. by edge embossing. Examples ofsuch papers are toilet paper and facial tissues. Also in this case, thesamples are cut so that a long side of the ply-bonding zone of eachsample will be perpendicular to the pulling direction.

[0071] The multi-ply sample 10 is fixed in the clamps 15 without anyvisible sagging and without the sample being under tension. The testsurface of the sample between the clamps may not be touched withfingers. When the sample is aligned correctly, the test is carried out.The ply bonding is measured on 50 mm wide, conditioned samples at a freetest span length 16 of 30 mm, see FIG. 7, with an elongation rate of 100mm/min. A 10N measuring cell is needed. The clamps shall be designed sothat there is no initial stress when the sample is fixed and the sampledoes not slip when load is applied. The upper clamp holding the sampleshould not weigh more than 10 g including holding pin, to maintain themeasuring cell's background noise at a low level. The test path is 100mm for full-surface bonded products. In the case of products with borderor edge embossing, the test of separating the plies is interrupted.

[0072] The individual plies of the two-ply or multi-ply test stripsshall separated on one end at a length of 2 cm. In a 3-ply product, forinstance, ply 1 is then separated from plies 2/3. The single ply is thenfixed in the lower clamp. Subsequently, plies 2 and 3 are separated fromeach other.

[0073] During the test, the end of the strip not fixed in the clampshould hang down freely or rest on a smooth, horizontal surface 17.

[0074] The results shall be calculated separately for machine directionand cross direction. The average value of the force maximum within themeasuring path and the mean value of the peaks over the total mean valueshall be calculated and stated in Newton: Fmax Lm: Force maximum withinthe measuring path Mean value Fmax Sp: Mean value of the peaks over thetotal mean value

EXAMPLE 1

[0075] Trials have been made according to this invention. Two plies ofthe base tissue for the Double Velvet base tissue from SCA HygieneProducts containing cellulosic virgin fibres was used. The base tissuehas a basis weight of 22 g/m². The two tissue plies was bonded byultrasonic bonding using the parameters stated below. Ultrasonic hornHerrmann Ultraschalltechnik Calender Eduard Küster Maschinenbau Bondingpattern See FIG. 8 Ultrasonic frequency  35 kHz Pressure force 400 NLine speed Trial 1: 50 m/min Trial 2: 300 m/min Pressure area 8.3% ofthe area of the ply-bonding zone Result Ply-bonding strength Fmax LMTrial 1: 0.163 Trial 2: 0.645 N/50 mm

[0076] These trials shows that it is possible to get a god ply-bondingby ultrasonic bonding of dry tissue paper plies.

EXAMPLE 2

[0077] Further trials according this invention have been done withtissue with or without lotion. Two plies of the base tissue for the ZewaSoft base tissue containing cellulosic virgin fibres from SCA HygieneProducts was used. The base tissue has a basis weight of 18 g/m². Thetwo tissue plies was bonded by ultrasonic bonding at different speeds.The same ultrasonic equipment as in example 1 was used with the sameultrasonic frequency and pressure force. Trial 1-3 was made withoutlotion. In trial 4, 3 g/m² of a lotion composition was then sprayed ontoone side of the bonded tissue paper. The plybonding strength wasmeasured before and after the lotion was applied.

[0078] The lotion composition used was by weight 5.3% polyglycerylpoly(12-hydroxy stearate)(PGPH), 5.3% lauryl glucoside, 3.0% glycerylestearate (Cutina®MD available from Cognis Deutschland GmbH, Tyskland),30% cocoglyceride (Myritol®331 available from Cognis Deutschland GmbH,Tyskland), 30% di-n-octyl carbonate (Cetiol® CC available from CognisDeutschland GmbH, Tyskland), 0.1% citric acid (citric acid is presentfor pH adjustment in the from Cognis Deutschland GmbH, Tysklandcommercially available emulsifier combination Eumulgin® VL 75, based onPGPH, lauryl glycoside, glycerol and water, which was used for preparingthe lotion, 1.5% bisabolol, 4.0% glycerol, Phenonip®(a from ClariantDeutschland, Tyskland commercially available preservative mixturecontaining phenoxyethanol as well as methyl-, ethyl- propyl- andbutylparaben) and water up to 100%.

[0079] The lotion composition had a viscosity of about 300 mPaxs at 23°C. (measured with a Brookfield-RVF viscosimeter, spindle 5, 10 rpm).Conductivity measurements showed that the above lotion is of O/W type.Plybonding strength Plybonding strength after Machine before lotionapplication lotion application Trial speed m/min Mean value Fmax SP MeanValue Fmax SP 1  50 0.041 2 100 0.141 3 150 0.256 4 200 0.182

[0080] As can be seen from example 2 the ply-bonding strength increaseswith the machine speed. Therefore it is assumed that the ply-bondingstrength before the lotion was applied to the tissue paper in trial 4 ishigher than for the tissue paper in trial 3. It can further be seen fromthe table that the addition of lotion does not weaken the ply-bondingstrength to a level below the wanted.

1. A method for autogenously bonding at least two plies of tissue paperwith a basis weight from 10 to 45 g/m² comprises the steps of laying theplies face-to-face, bonding the plies to each other with the help ofultrasonic means having at least one ultrasonic horn which oscillatesperpendicular to the surface of the tissue paper plies
 2. The methodaccording to claim 1, wherein dry tissue paper plies are bonded.
 3. Themethod according to claim 1, wherein the tissue paper comprises purelycellulosic fibres.
 4. The method according to claim 1, wherein at leastone of the plies is chosen from the group of dry-creped tissue paper,through-air-dried tissue paper and impulse embossed dried tissue paper5. The method according to claim 1, wherein the at least one hornoscillates with a frequency from 15 to 50 kHz.
 6. The method accordingto claim 1, wherein the bonding step results in a ply-bonding zone, anda bonding area defined as the sum of all areas of the bonds in saidply-bonding zone, said bonding area ranging from 0.5 to 50% of the areaof the ply-bonding zone.
 7. The method according to claim 6, wherein thebonding area ranges from 2 to 40% of the area of the ply-bonding zone.8. The method according to claim 6, wherein the bonding area ranges from5 to 20% of the area of the ply-bonding zone.
 9. The method according toclaim 1, wherein the pressure force between the at least one horn andthe tissue paper plies to be bonded is from 50 to 700 N.
 10. The methodaccording to claim 1, wherein the pressure force between the at leastone horn and the tissue paper plies to be bonded is from 200 to 500 N.11. The method according to claim 1, wherein the paper plies areultrasonically bonded while they are transported at speed from 50 to1800 m/s.
 12. The method according to claim 1, wherein the paper pliesare ultrasonically bonded while they are transported at speed from 300to 800 m/s.
 13. The method according to claim 1, wherein lotion is addedto the bonded surface.
 14. The multi-ply tissue paper, each ply of thetissue paper having a basis weight from 10 to 45 g/m², wherein saidmulti-ply tissue paper is bonded autogenously by ultrasonic means. 15.The multi-ply tissue paper according to claim 14, wherein the sum of theareas of the bonds is from 0.5 to 50% of the area of the ply-bondingzone.
 16. The multi-ply tissue paper according to claim 14, wherein thesum of the areas of the bonds is from 2-40% of the area of theply-bonding zone.
 17. The multi-ply tissue paper according to claim 14,wherein the sum of the areas of the bonds is from 5-20% of the area ofthe ply-bonding zone.
 18. The multi-ply tissue paper according to claim14, wherein the tissue paper comprises purely cellulosic fibres.
 19. Themulti-ply tissue paper according to claim 14, wherein at least one ofthe plies is chosen from the group of dry-creped tissue paper,through-air-dried tissue paper and impulse embossed dried tissue paper.20. The multi-ply tissue paper according to claim 14, wherein the tissuepaper contains a lotion.